1. Field of the Invention
This invention relates to optical pumps for compact high power lasers and in particular, to high power optical pumps including Vertical Cavity Surface Emitting Laser (VCSEL) and arrays of VCSELs, for operating high power lasers in continuous wave (CW), Quasi Continuous Wave (QCW) and pulse operation modes.
2. Description of the Related Arts
Optical pumping is one of the preferred methods to operate a class of high power lasers. A high power laser typically comprise of a gain medium enclosed in a resonant feedback cavity having at least one high reflectivity surface and one partially reflective surface placed on opposite ends of the cavity, such that laser emission is emitted out of the partially reflective surface. The laser light in general is emitted along the length of the gain medium to be referred as the laser axis hereinafter for the ease of description. The gain medium may be in the form of a solid rod for example in a solid state laser, a semiconductor laser, a fiber such as an optical fiber, or a gas, liquid or a gel, either confined in a tube or an enclosure or circulated in a closed path.
In a typical high power laser, the gain medium may be pumped from one or more sides of the gain medium that are perpendicular to the laser axis using one or more high power flash lamps. In operation, optical energy from a flash lamp excites the gain medium atoms/molecules to populate a higher atomic level and creates a population inversion, a condition necessary for lasing. When the excited atoms fall back to their ground state spontaneously, photons of a specific energy (or emission wavelength) are generated. The photons so generated undergo multiple reflections between the reflective surfaces of the resonant cavity thereby, amplifying the light generated in the laser cavity that is emitted out of the cavity as laser emission. The intensity of laser emission or the output power from a laser is directly proportional to the pump power.
As the gain medium temperature rises during lasing operation, the absorption spectrum of the gain medium shifts. High power flash lamps are widely used because their emission spectrum is broad and does not require wavelength tuning to match the absorption spectrum of the laser gain medium. However, broad emission spectrum of flash lamps also is a major disadvantage because wavelength range for absorption in typical gain media is limited. Therefore, a large fraction of the optical energy from the flash lamp is not utilized effectively in pumping the gain medium and part of the pump energy is transferred as heat in the gain medium giving rise to undesirable effects including thermal lensing. Since typical gain media comprise of material that exhibit poor thermal conductivity, temperature variations in the gain medium may cause instabilities that are detrimental for high power operation in continuous wave (CW) mode, quasi-continuous mode (QCW) or in pulsed mode for example, in Q-switch mode used in applications such as wavelength conversion. Temperature instabilities may even affect the polarization properties. Therefore, to achieve stable lasing operation, elaborate cooling equipment is necessary for the laser as well as the optical pump, for stable operation particularly at high output power.
One way to mitigate some of the drawbacks of flash lamps is to use high power semiconductor lasers which have a definite edge over flash lamps. For example, semiconductor lasers may be designed for emission in a specific narrow wavelength range may be tuned specifically to match the absorption spectrum of the gain medium. Semiconductor lasers require relatively low electrical drive current as compared to flash lamps. Due to their small footprint, semiconductor lasers may be cooled using relatively small thermoelectric coolers and may be stabilized for a fixed wavelength emission for extended periods.
Two different types of semiconductor lasers namely, edge emitting laser (EEL) and vertical cavity surface emitting laser (VCSEL) have been used for optical pumping of gain media in high power lasers. While individual semiconductor lasers may not have high output power, they may be arranged in one-or two-dimensional array depending upon the type of semiconductor laser, to increase total pump power. Since semiconductor lasers or laser arrays are small, they can be placed very close to the gain medium or may even be integrated with the gain medium, in particular for a VCSEL or a VCSEL array pump. Semiconductor lasers or laser arrays may be arranged for optically pumping a gain medium from a side(s) or an end. For example, if the gain medium is in the form of a rod, in a side pumping configuration the pump power is coupled along the side(s) parallel to the length the rod. The optical pumps in this configuration are positioned along the length of the gain medium such that the light from the pump is incident perpendicular to the laser axis. In an end pumping arrangement, the pump power is coupled to the cross section at one end of the rod. Both methods of optical pumping have their merits and are described in many patent and non-patent publications.
For example, in the U.S. Pat. No. 5,315,612 issued on May 24, 1994, to Alcock et.al. a solid state laser cavity suitable for side pumping a solid state gain medium has been described. More specifically, the gain medium is pumped using an optical pump including a semiconductor laser bar comprising a large number of edge emitting lasers stacked in a one-dimensional array. The cavity design disclosed there may be used with or without collimating optics between the optical pump and the solid state gain medium.
In the U.S. Pat. No. 5,455,838 issued on Oct. 3, 1995, Heritier et.al, described a laser device by optically pumping a gain medium having rectangular cross section, using pump light from one-dimensional array of edge emitting semiconductor lasers. In this arrangement pump light is provided from the side on two opposing flat surfaces of the gain medium using cylindrical lenses attached to the gain medium surface for uniform pump light distribution along the length of the gain medium.
In the U.S. Pat. No. 5,978,407 issued to Chang et al on Nov. 2, 1999, an optical pumping arrangement using a one-dimensional arrays of edge emitting semiconductor lasers is described where a specially designed light diffuser is used to direct the pump light onto sides of a circular cross section solid state gain medium rod. The pump light traverses through light paths constructed in a specially designed segmented cylindrical light coupling structure built around the cooling arrangement of the gain medium rod.
In the U.S. Pat. No. 6,157,663 issued on Dec. 5, 2000, Wu et al. describe an optical pumping configuration to side pump an Nd:YVO4 gain medium using an array of edge emitting semiconductor lasers. The pump laser array is positioned to side pump the gain medium without collimating or focusing optics. A gap between the pump laser array and the gain medium is empirically selected to match the angular extent of the pump laser output light to the height of the gain mode at the position of gain mode fixed to optimize coupling and diffraction losses.
While it is well established that edge emitting semiconductor lasers and in particular, one dimension array of such lasers provide adequate optical power to pump a solid state laser, they have some limitations. For example, emission from an edge emitting laser is elliptical and highly astigmatic having greater than 20 degrees half angle, in one dimension and a divergence of about 10 degrees in the other. Therefore, emission from an edge emitting laser requires a complex arrangement of optical components to generate a well focused narrow beam for pumping a gain medium in end-pumping configuration. In particular, in the U.S. Pat. No. 6,157,663 issued on Dec. 5, 2000, Wu et al. several limitations of end-pumping configuration in the context of a solid state gain medium has been summarized.
Different configurations for end-pumping a gain medium have been described in several patent and non-patent publications. In a publication entitled “Efficient Nd:YAG laser end pumped by a high-power multistripe laser-diode bar with multiprism array coupling” Applied Optics, vol. 35, No. 9, 1996, pp. 1430-1435, Yamaguchi et al. described a multiprism array to couple pump light from a high power edge emitting laser array bar to a Nd:YAG gain medium. An alternative method using optical fiber to couple pump laser output from edge emitting lasers or laser arrays has been described in several other non-patent publications.
In an article entitled, “Characteristics of laser operation at 1064 nm in Nd:YVO4 under diode pumping at 808 and 914 nm” published in the Journal of Optical Society of America B, Vol. 28, No. 1, 2011, pp. 52-57, Délen et al. described using a large lens to focus pump light from two lasers onto a Nd:YAG gain medium. Other end-pumping configurations using optical fiber coupling from laser array or individual lasers have been described in “Nd:YAG laser diode-pumped directly into the emitting level at 938 nm” by Sangla et al, in Optics Express, Vol. 17, No. 12 8, Jun. 8, 2009, pp. 10091-10097, and “3.5-W Q-switched 532-nm Nd:YAG laser pumped with fiber-coupled diode lasers” by Hemmati et al. in Optics Letters, Vol. 19, No. 17, Sep. 1, 1994, pp 1322-1324.
Although pumping a gain medium in side-as well as end-pumping configurations has been demonstrated, edge emitting lasers have certain limitations. A high level of temperature dependence (about 0.3 nm/deg° C.), which necessitates stringent temperature control of the edge emitter laser pumps to prevent pump wavelength from drifting away from the absorption wavelength of the gain medium to avoid creating a mismatch between them. In order to stabilize emission wavelength edge emitting laser requires a dedicated thermoelectric cooler or micro-channel cooler which are cooled by liquid from a refrigeration system called chiller. In arrays of edge emitting lasers, the minimum distance that is allowed between each laser may be limited by the space required to accommodate the cooling arrangement. As a result, the maximum size of an array is determined by the number of lasers that can be placed within a desired physical space. It is also inconvenient to configure edge emitting lasers into a two-dimensional array as it requires complex assembly of edge emitter bars into stacks, as is well known in the art.
Therefore, as an alternative, VCSELs have been utilized for pumping a gain medium. A VCSEL emits a symmetrical beam having a circular cross section and a typical half and full divergence angle of 9 and 20 degrees, respectively, in both dimensions (FIG. 1 showing data from a device constructed at Princeton Optronics Inc., Mercerville, N.J.). As a result, VCSELs are easily adaptable to simple optical methods for generating or modifying the output light for a desirable illumination pattern using co-linear focusing optics which is particularly convenient for end-pumping configuration. A particularly useful optical pump configuration using a microlens integrated with a VCSEL or a VCSEL array, for applying pump power at one end of a gain medium directly or via a fiber light path, is described in the U.S. Pat. No. 6,888,871 issued to Zhang et al. on May 3, 2005 and co-owned by the assignee of this application. The content of the above mentioned patent is being incorporated by reference in its entirety.
In an article entitled “VCSEL end-pumped passively Q-switched Nd:YAG laser with adjustable pulse energy” published in Optics Express, Vol. 19 No. 5, February 2011, pp. 4261-4267, Goldberg et al. describe optical pumping of a solid state gain medium using a VCSEL array. The authors of this article acknowledge that VCSEL array described therein is developed at the Princeton Optronics Inc. who is also the assignee of this application.
In the U.S. Pat. No. 7,949,002 issued on May 24, 2011, Miesak et al. describe an optical pumping method including an array of VCSEL in a slab of laser gain medium. More importantly, the optical power is coupled to a gain medium without any coupling optics. The VCSEL array is mounted directly on the gain medium so that the gain medium and the VCSEL array share a cooling system. One advantage of this method is that increased cooling of the VCSEL array facilitates higher output power from the pump which is directly coupled to the gain medium. This approach has disadvantages however when increased pump energy is required.
One disadvantage of the method described in the Miesak patent is related to a practical limit to the size of a VCSEL array due to fabrication limitations. Thus for higher pump powers several VCSEL arrays are needed to be placed adjacent to each other. As a result there are undesirable gaps in the incident pump beam on the laser rod. To overcome this problem a diffuser plate is used to redistribute the energy more evenly along the laser rod. This diffuser adds losses to the pump beam since light is diffused away from the laser rod in some areas. An additional disadvantage is that the maximum size of the VCSEL arrays is constrained by the size of the side surface.
A second disadvantage relates to the cooling of the laser rod and the VCSEL devices. For high power lasers, cooling is required for both the laser rod and the VCSEL arrays. Locating the VCSEL arrays so close to the laser rod increases the complexity of the cooling arrangement. This is especially the case with the VCSEL arrays since they must be protected from any cooling liquid degrading the optical emitting surfaces and the electrical contacts.
A different arrangement for optically pumping a solid state gain medium is described in the U.S. Pat. No. 7,430,231 issued to Luo et al. on Sep. 30, 2008. A specially designed diffusion chamber is used to couple incoherent or partially coherent light from a VCSEL array. Light from the VCSEL arrays are coupled via slits along the length of the chamber. The slits are evenly distributed on the periphery of the diffuser chamber. In an alternative method, a compound parabolic reflector chamber is used to couple pump light to a gain medium placed at the focal point of the parabola.
There are two disadvantages of this approach. The reflection cylinder and diffusion material will cause a small amount of loss at each reflection and diffusion due to imperfections and other absorption effects. The losses from the many reflections and scattering in these components can reduce the pump light absorbed in the laser rod. The second disadvantage is that the laser beam intensity cross-section is not uniform but follows a more Gaussian type of distribution being highest intensity in the center and weaker intensity at the edges. A more efficient pumping energy distribution would be one that matches this distribution so that the highest activation power for the active ions is in the center of the solid-state laser beam.
In another non-patent publication entitled “High Power 808 nm VCSEL arrays for pumping of compact pulsed high energy Nd: YAG lasers operating at 946 nm for blue and UV light generation” published by SPIE in February 2011, an optical pump module configured for side-pumping a solid state laser gain medium is described by van Leeuwen et al., some of the authors of that paper are also applicants in this application. The content of this publication is being incorporated by reference in its entirety.
Being a surface emitting device, VCSELs are more amenable to integration with a solid state gain medium. For example, in the U.S. Pat. No. 5,796,771 issued on Aug. 18, 1998, Den Baars et al. describe a solid state laser integrated with a single VCSEL pump laser that is grown on a common substrate. In another U.S. Pat. No. 5,982,802 issued on Nov. 9, 1999, Thony et al. describe a solid state laser that is pumped by a single VCSEL device or a two-dimensional array of VCSEL devices including a microlens.
The optical pump configurations described in the prior art publications mostly disclose the use of bars of edge-emitting semiconductor laser arrays, a single VCSEL device or arrays of VCSELs, for optical pumping in a side-pumping configuration. In fact, none of the prior publications describe a system that is adaptable to side-pumping and end-pumping configuration. Furthermore, most of the prior art publications describe optical pumping methods that are particularly suited for solid state gain medium and do not disclose arrangements for other types of gain media for example, semiconductors, liquid or gel, etc. In addition, none of the prior art publications teach or suggest a pumping method that is readily adaptable for upgrading the pump power in a modular fashion.
Contrary to the prior art publications, this invention describes an optical pumping system that is adaptable for end-pumping as well as side-pumping configurations utilizing unique properties of high power VCSEL devices developed at Princeton Optronics Inc. In this invention optical pump configurations using a single VCSEL or VCSEL arrays adaptable for side pumping and end-pumping modes are provided for high power laser operations. More importantly, the optical pump systems to be described shortly may be in close proximity to the laser gain medium without interfering with the laser cavity design of the gain medium. Therefore, the pump modules provided in this invention are adaptable to work with different types of gain media used in different high power lasers including but not limited to, solid state lasers, semiconductor lasers, gas lasers, liquid or gel dye lasers, fiber lasers, etc. The optical pumping system may be constructed to conform to different shapes of the gain medium and may be modular for quick adaptation to upgrade output power to pump different volume (size) of gain media. In addition, the optical pump modules described in this invention are adaptable to emit uniform pump light to match different cross section geometry of the gain medium. The principles may be applied to construct compact high power lasers for a variety of applications.